Impact actuator

ABSTRACT

Actuators capable of repeated impacts in which a hydraulically driven follower follows the expansibly driven impact member. Also shown are constant input and output hydraulic flows valved alternately between an impact member - returning motion which recompresses the expansible drive, and a following motion causing the follower to follow the suddenly accelerated impact member. The follower is shown to control these flows by means of a slave valve operated by changes of pressure attributable to the position of the parts. In the linear actuator embodiments shown a stationary tube between the linear piston and the moving follower sleeve separates the follower drive chamber from the expansible medium in one illustrated embodiment and a follower drive chamber formed along the side of the piston is featured in another illustration.

United States Patent [1 1 Peterson 1 Dec. 16, 1975 [21] Appl. No.:322,110

[52] U.S. C1. 60/371; 91/165; 91/231;

91/309; 91/313; 91/321; 91/402; 92/134 [51] Int. Cl. FOIL 25/06; FOIB7/18 [58] Field of Search 91/230, 231, 235, 313,

[56] References Cited UNITED STATES PATENTS 1,429,786 9/1922 Smith91/231 1,626,087 4/1927 Hultquist 91/231 2,997,849 8/1961 Shimanckas..91/402 3,411,592 11/1968 Montabert.... 91/321 3,524,385 3/1970 Ottestad91/235 3,583,158 6/1971 Foster 91/235 3,595,133 7/1971 Foster 91/3903,687,008 8/1972 Densmore 91/276 3,735,823 5/1973 Terada 173/119DISCHARGE Fisk 92/134 Phillips 91/165 Primary Examiner-Paul E. Maslousky5 7] ABSTRACT Actuators capable of repeated impacts in which ahydraulically driven follower follows the expansibly driven impactmember. Also shown are constant input and output hydraulic flows valvedalternately between an impact member returning motion which recompressesthe expansible drive, and a following motion causing the follower tofollow the suddenly accelerated impact member. The follower is shown tocontrol these flows by means of a slave valve operated by changes ofpressure attributable to the position of the parts. In the linearactuator embodiments shown a stationary tube between the linear pistonand the moving follower sleeve separates the follower drive chamber fromthe expansible medium in one illustrated embodiment and a follower drivechamber formed along the side of the piston is featured in anotherillustratron.

19 Claims, 7 Drawing Figures U.S. Patent Dec. 16,1975 Sheet10f43,925,985

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US. Patent Dec. 16, 1975 IMPACT ACTUATOR FIELD OF INVENTION The presentinvention relates to impact delivering devices as might be used for rockdrilling, pavement breaking, forging, pile driving, impact wrenches,etc., and in particular to such devices as powered by high pressurehydraulic fluid.

BACKGROUND OF INVENTION Actuators capable of delivering repeated impactsto a variable position, whether in straight or rotary motion, are known.Those to which the present invention relates are of the type in which anelastic or expansible drive, e.g. a trapped volume of pre-compressed airor a spring, is exposed to the impact member to drive it. Countervailinghydraulic pressure controlled by the follower is used in the cycle toreturn the impact member and recompress the expansible drive prior toinitiation of the next impact. It is known to return the follower withthe impact member and to employ automatic triggering arrangements. Onesuch known arrangement involves venting of the hydraulic return fluidwhen the impact member reaches the return position' whereby restraintupon the expansible drive is suddenly removed.

While operable to advantage, such known devices have involved large andexpensive components, undesirable limitations on impact rate and design,and other drawbacks which it is the object of the present invention toimprove upon.

BRIEF DESCRIPTION OF FEATURES AND ADVANTAGES OF THE INVENTION Theinvention features use of hydraulic drive for the follower as it followsthe impact member, and use of a hydraulic system including a positivedisplacement constant flow rate source of hydraulic fluid, whichalternately is caused to drive the impact member in its return directionand the follower in its following direction.

Preferred embodiments of the invention feature: a trigger effective torelease the impact member and actuate the separate motion of thefollower, preferably the trigger in the form of a hydraulic vent; use ofreturn drive pressure to hold a directional slave valve to vent thefollower drive while causing the return of the impact member; andemploying the trigger vent-reduced pressure to reverse the valves, toapply pressure to the follower while venting the impact member returnsystem. Preferred embodiments feature a linear actuator construction ofsimple design. Other features are mentioned in the Abstract.

Regarding advantages of the present invention it is noted that certainprior through-flow hydraulic impacting devices, including one inventedpreviously by the present inventor, have had very unsteady flow,particularly in the discharge line where flow rate may vary between zeroand double or more the average value. Such devices use expansible means(e.g., compressed gas) to restore a follower or sleeve to a downwardposition to begin a subsequent cycle. According to the present inventionI have realized that such construction are un-needfully limited inefficiency (energy expended by the elastic member in moving the followersleeve is wasted) and in descent rates and, hence, blow frequency. A gaschamber, used as an energy storage device, must remain at pressure belowhydraulic fluid pressure in such devices.

In contrast the present invention can achieve nominally constant flowrates in both inlet and discharge lines at all times. Its sleeve orvalving unit, being positively driven in both directions, permits higherfrequency operation. It may utilize gas or other energy storagemechanisms at pressures higher than hydraulic fluid pressure, thusallowing a smaller energy storage device or chamber.

These and other objects, features and advantages will be understood froma description of a preferred embodiment taken in conjunction with thedrawings wherein:

FIG. 1 is a cross section of the presently preferred embodiment of theinvention.

FIGS. 2a-d are similar cross-sections at different times in theoperating cycle.

FIG. 3 is a similar cross-section for the special case of pistonovertravel.

FIG. 4 is a partial view of an alternate construction of the top of theunit of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Referring to FIG.1, the device consists of a housing 11, a piston 10, and a sleeve 12. Asshown the piston is generally rod-like and of constant diameter exceptfor an enlargement 10a in the center. A relatively thinwalled tube 13,extends from the housing and its inner surface makes a sliding seal withthe upper portion of the piston. Gas is stored in chamber 14 defined bythis tube 13, the housing 11, and the top of the piston 10. Gas chargingvalve is shown at 15, where gas is injected into chamber 14 at intervalsto make up for leakage from chamber 14. The movable sleeve 12, slidinglyseals on the outer surface of tube 13 and the inner surface of housing11. The sleeve 12 is flared at 12a at its lower end and shaped to sealagainst the piston enlargement at 16. The sleeve contains a number oflongitudinal internal holes 17, communicating radially through shortholes 17a with its outer surface at 18. A number of external short,axially extending slots, 19, are provided in the sleeve in such positionas not to communicate with holes 17. The piston 10, piston enlargement10a, sleeve 12, and tube 13, define an annu lar volume of fluid at 20.

The housing 11 includes an upper bore 11a, sized to seal slidingly withthe sleeve 12 and a second, next lower bore 11b of enlarged diametermaking no seals with any moving parts. A third, next lower bore 110, ofshort axial dimension, again is sized to seal slidingly with the sleeveand defines with the outer surface of sleeve 12 and upper parts of thehousing an annular volume 22.

Below bore the housing has a bore lle of larger dimension than thoseabove, defining a region for free axial movement of the enlargement 10aof the piston, and of the flared end 12a of piston 12, with ample spaceS for displacementof fluid about the periphery of these members whenthey move.

The lower-most bore 11f of the housing slidingly seals against thepiston.

The housing contains a lower port, 23, communicat ing with the enlargedlower housing volume 23", a vent, 24, communicating with the volume 22,and an upper port 25, communicating with volume 15'', the volume abovethe sleeve 12.

The volume 14 is filled with gas to the desired pressure. Except forleakage, this operation is not repeated.

Referring to FIG. 2a, the unit is combined with an ordinary two-positionreversing valve shown schematically at 26. It contains an internalshuttling member or spool 27 that is biased upwardly by springs at 28.The valve is connected to a pump 29 of the positive displacement,constant flow type, through line 30, and to a return reservoir (notshown) through line 31. Opposite ends of the valve spool 27 arepressurized from these two lines through lines 30' and 31'. The valve isconnected to the hammer as shown with line 23 to port 23 and volume 23",line 25 to port 25 and volume 25". Vent 24 is connected through line 24'to the reservoir line 31.

In operation, starting with the position of FIG. 2a, similar to FIG. 1,inflow from the pump is entering volume 23". Volume 20, between sleeveand piston, is at low pressure, sealed from 23" at 16 (FIG. 1) andvented through holes 17, 17a, 18, volume 22, port 24 and line 24 to thereservoir. Volume 25 is also vented to the reservoir. The outer diameterof the main body of sleeve 12 is less than the diameter of the seal at16, hence there is a net downward force acting upon the upper surface ofthe flare 12a, holding the sleeve to the piston as suggested by arrowsat 32. High pressure inflow at 23' to volume 23" then pushes the pistonand sleeve combination upward as a unit, venting fluid to discharge fromvolume 25" and from annular volume 20. This same high pressure of thepump fluid (determined by the resistance of the gas pressure in volume14 and the appropriate piston areas) acts through line 30' to keep valve26 in the down position shown, with spring 28 compressed. Upward motionof piston continues until the position shown in FIG. 2 b is reached.Here, slots 19 short circuit the sea] at the bottom of volume 22allowing flow from main volume 23" into volume 22 and thence into volumeand out vent line 24'. The net effect is a reduction in pressure involume 23" and an increase in pressure in volume 20 until the net forceacting across piston enlargement 10a is no longer sufficient to hold thepiston 10 upward against gas pressure in volume 14, albeit sleeve 12still moves upwardly due to pressure differential across it caused bycontinued inflow to volume 23". The seal at 16 is thus broken and thepressure differential across enlargement 10a is completely destroyed.The piston is thus triggered" and free to move downward rapidly underthe influence of compressed gas in volume 14, here without anydisplacement of hydraulic fluid from volume 23" because the two ends ofthe piston are of equal diameter. Once seal 16 is broken, pressures involumes 23 and 25" tend to equalize across the equal area ends of sleeve12. Thus pressures in lines and 31 become equal, at vent pressure, as dopressures in lines 30 and 31, so that the valve spool 27 switches to theposition shown in FIG. 2b under the influence of spring 28. Then as inFIG. 20, inflow is directed into volume 25", pushing sleeve 12 downward(the piston 10 having already descended rapidly and delivered its blow).Downward motion of sleeve 12 pushes fluid out of volume 23 through line23' (some will also exit through holes 17 and line 24' Outflow throughline 31 will equal inflow through line 30. Because the frontal area ofsleeve 12 is less than the effective upstroke piston area in thisembodiment (by the frontal area of tube 13), the downward velocity ofsleeve 12 will, using the same flow rate in the two different phases, be

greater than the upward velocity of the piston-sleeve combination.Pressures in volumes 23" and 25 during downstroke of sleeve 12 will beabout equal and valve 26 will remain as shown in FIG. 20 for the entiredownstroke.

When the sleeve 12 seats downwardly against the piston as in FIG. 2d itcan descend no further (e.g. a bit at the lower end of the piston beingfirmly in contact with the rock by virtue of hold-down force on theentire unit). Fluid can no longer enter volume 25 and having no placeelse to go, it will flow through line 30, rising in pressure asnecessary to displace the spool 27 and compress spring 28. The sleeve isthus firmly seated on the piston, sealing at 16, with volume 20 returnedto low pressure. With the valve switched as in FIG. 2d, then, flowenters volume 23" where, because of resistance to upward movement ofpiston and sleeve by residual gas pressure in 14, it immediately assumesa relatively high pressure thus keeping valve 26 in the position shownfor the upward stroke. The cycle then repeats.

FIG. 3 illustrates the special case when the bit 50 at the end of thepiston has dropped away and the piston overtravels to the positionshown, stopped by a mechanical stop (with snubbing action) shownschematically at 40. In most applications it is desirable to have thepiston stop cycling when this occurs. This can be provided as shown byhaving the top of sleeve 12 travel below the top of volume 22, as at 41,before it travels sufficiently downward to make a seal at 16 with thenow depressed piston. Inflow at 25 will now flow out vent 24 and alsothrough holes 17, volume 20, the open seal 16, volume 23", and line 23.There will thus be no pressure build up in volume 25" and the valve 26will not switch. To restart, the bit 50 must be pushed relatively backup against the piston (i.e., the housing lowered) sufficiently to make aseal at 16 and close the gap at 41. Pressure in volume 25" will thenbuild up and the valve 26 will switch, starting a new cycle.

Referring to the embodiment of FIG. 4, an integral housing casting 11'which forms the bores 11a and 1 lb, etc., has an uppermost bore 11gabove and of smaller diameter than bore 11a. A constant diameter portionof piston extends entirely through sleeve 12 into sealing contact withbore 11g throughout the operating range, forming compressed gas chamber14. By this construction the necessity of sleeve 13 of FIG. 1 isavoided.

The dashed lines of FIG. 4 illustrate an automatic filling arrangementuseful when volume 14 is sized to receive hydraulic fluid to provide ahydraulic spring for driving the impact member. For filling or forrefilling to make up for loss, the upper dashed line receives fluid fromthe pump (line 30) through check valve 14. The lower dashed line leadsto vented discharge. This vent is positioned just below the piston topedge when the piston is at its normal impact point. This will enablevolume 14 to be vented as the piston returns from an abnormally lowposition (as in FIG. 3), and thereby sets the maximum amount ofhydraulic fluid trapped in volume 14, and thus avoids build-up of excesspressure in volume 14 during operation. The construction shown in FIG.4, in which volume 14 is formed by the housing, can be made veryruggedly and is suitable for the pressures associated with use ofhydraulic fluid as the spring for driving the impact member.

Instead of these constructions for instance: a radially floatingseparately formed stationary tube 13 could be employed; instead ofcompressed gas other compressible material or mechanism could act uponthe piston or hydraulic fluid as just mentioned; the ends of the pistoncould be of different area with provision for flow of displaced fluids;the entire reciprocating structure could be of rotary rather than lineararrangement; all fluid can be made to exit chamber 23 through hole 17and port 24, etc.

These and other variations in the specific details shown are within thespirit and scope of the following claims.

What is claimed is:

1. In an actuator capable of delivering repeated impacts to a variable,stopped position comprising the combination of an impact member movableback and forth relative to said position, an expansible drive fordriving said member toward said position while expanding, and a returnfor returning said impact member and recompressing said drive, saidreturn including a follower which follows said impact member to itsvariable stopped position and a hydraulic system enabled by saidposition of said follower to apply return force to said impact member,the improvement wherein said hydraulic system includes a hydraulic drivefor said follower in the following direction, a valve separate from saidfollower and impact member effective to direct said flow alternately todrive said follower in following direction and drive said impact memberin its return direction and a control for actuating said follower driveupon said driving of said impact member.

2. The actuator of claim 1 wherein said hydraulic system includes asource of hydraulic fluid which comprises a positive displacementconstant flow pump and said impact member has a constant cross-sectionat its opposite end regions whereby movement of said impact member doesnot displace hydraulic fluid.

3. The actuator of claim 1 including a trigger, said follower beingreturnable by said hydraulic fluid with said impact member, and saidtrigger being effective both to release said impact member to be drivenby said expansible drive and to actuate said control for separatefollowing motion of said follower drive.

4. The actuator of claim 3 wherein said trigger comprises a vent openedby said impact member and follower reaching said return position, saidvent operable both to relieve the net hydraulic return pressure uponsaid impact member to the point where it is overcome by said expansibledrive and to actuate said control.

5. The actuator of claim 4 wherein said valve is held in a firstposition by pressure returning said impact member, the valve effectiveto admit return fluid to said impact member and to vent said followerdrive, and said vent effective, through relief of return pressure, tooperate said valve to a second position to apply hydraulic pressure toand control said follower drive.

6. The actuator of claim 5 wherein said vent is also effective to open afurther vent of said impact member return.

7. The actuator of claim 5 wherein said valve is operative in saidsecond position to apply the pressure of said follower drive to saidvalve upon buildup of pressure in said follower drive associated withsaid follower reaching said impact member, said pressure operative toreverse said valve to said first position.

8. The actuator of claim 4 wherein said trigger is formed by matingportions of said follower and impact member providing a seal when seatedand creating pressure conditions whereby said impact member and followerare returned as a unit, and in the return position said vent operativeto break said seal and expose a surface of said impact member to forcesconcelling return forces.

9. The actuator of claim 8 wherein said follower and said impact memberwhen seated together define a fluid volume, said fluid volume ventedthrough a passage in said follower to a low pressure region, saidfollower having a surface portion exposed to hydraulic pressureeffective to press said follower and impact member together and preventrelief through said passage of pressure acting upon said impact member,and said vent operative to introduce hydraulic pressure to said fluidvolume tending to break said seal.

10. The actuator of claim 1 wherein said follower telescopes upon saidimpact member in response to said follower drive.

11. The actuator of claim 10 in the form of a linear actuator whereinsaid impact member comprises a reciprocable piston having an enlargedmiddle portion, said follower comprises a sleeve linearly telescopicallyfitted about the exterior of said piston, having a flared end engageableupon said enlarged middle portion of said piston and having anoppositely directed surface exposed to said hydraulic follower drive.

12. The actuator of claim 11 including a housing forming a first chamberinto which said piston and flared end of said sleeve extend, a firsthydraulic line connected thereto for said hydraulic return, a secondchamber to which said oppositely directed surface of said follower isexposed, a second hydraulic line connected thereto for said followerdrive, a third chamber intermediate said first and second chambers andnormally sealed therefrom, a third hydraulic line connected thereto forventing, a passage extending from said third chamber to the spacebetween said flared end of said sleeve and said enlarged portion of saidpiston, and a vent passage in the exterior of said sleeve positioned sothat when the sleeve is in its return position, said first chamber isvented to said third chamber.

13. The actuator of claim 12 including a positive displacement, constantflow hydraulic pump and a valve effective alternately to direct flow tosaid second line and exhaust from said first line and vice versa.

14. The actuator of claim 13 wherein a branch from the outlet of saidpump is connected so that pressure therefrom urges said valve toward afirst position directing said flow to said first line and exhaust fromsaid second line, said valve constructed so that relief of said pressureon said branch line causes said flows to be reversed.

15. The actuator of claim 1 in the form of a linear actuator whereinsaid impact member comprises a linear piston, a stationary tube in whichthe inner end of said piston is fitted, the inner end of said piston andthe interior of said tube cooperating to form a volume for an expansiblegas drive and said follower comprising a sleeve slidably engaged uponthe exterior of said tube and forming therewith a hydraulic chamber forsaid follower drive.

16. The actuator of claim 1 in the form of a linear actuator whereinsaid impact member comprises a linear piston, said follower comprising asleeve slidably engaged upon the exterior of said piston, said housingand a portion of the side of said piston forming a hydraulic chamber forsaid follower drive, said piston extending beyond said chamber in sealedrelation into a further chamber, the end of said piston disposed in saidfurther chamber and exposed therein to an expansible fluid drive.

17. The actuator of claim 1 in the form of a linear actuator whereinsaid impact member comprises a linear piston including a housingdefining a bottoming position for said impact member in the absence of aworkpiece at the impactreceiving position, said follower and housingcooperatively constructed to vent said follower drive prior to saidfollower reaching said impact member in said bottoming position, therebypreventing said follower from reaching the position for enablingapplication of return forces to said piston.

18. The actuator of claim 16 wherein said further chamber is connectedto a supply of makeup fluid through a check valve that permits inflowbut not outflow to said further chamber, and a'vent line from saidfurther chamber that is exposed when said impact member travels beyond aposition predetermined to limit the maximum fluid trapped in saidfurther chamber.

19. An actuator capable of delivering repeated impacts to a variableposition comprising the combination of an impact member movable back andforth relative to said position, an expansible fluid drive for drivingsaid member toward said position while expanding, and a return forreturning said impact member and recompressing said drive, said driveconsisting of a volume of fluid connected through a check valve to asupply of makeup fluid to allow inflow to said volume of fluid but notoutflow from said volume of fluid, and a vent from said volume of fluidopened by said impact member upon overtravel beyond a predeterminedposition and closed by said impact member upon return travel back tosaid predetermined position said vent thereby being adapted to ventexcess fluid at low pressure before it is recompressed by return motionof said impact member whereby said vent is effective to limit themaximum quantity of fluid trapped as said volume of fluid.

1. In an actuator capable of delivering repeated impacts to a variable,stopped position comprising the combination of an impact member movableback and forth relative to said position, an expansible drive fordriving said member toward said position while expanding, and a returnfor returning said impact member and recompressing said drive, saidreturn including a follower which follows said impact member to itsvariable stopped position and a hydraulic system enabled by saidposition of said follower to apply return force to said impact member,the improvement wherein said hydraulic system includes a hydraulic drivefor said follower in the following direction, a valve separate from saidfollower and impact member effective to direct said flow alternately todrive said follower in following direction and drive said impact memberin its return direction and a control for actuating said follower driveupon said driving of said impact member.
 2. The actuator of claim 1wherein said hydraulic system includes a source of hydraulic fluid whichcomprises a positive displacement constant flow pump and said impactmember has a constant cross-section at its opposite end regions wherebymovement of said impact member does not displace hydraulic fluid.
 3. Theactuator of claim 1 including a trigger, said follower being returnableby said hydraulic fluid with said impact member, and said trigger beingeffective both to release said impact member to be driven by saidexpansible drive and to actuate said control for separate followingmotion of said follower drive.
 4. The actuator of claim 3 wherein saidtrigger comprises a vent opened by said impact member and followerreaching said return position, said vent operable both to relieve thenet hydraulic return pressure upon said impact member to the point whereit is overcome by said expansible drive and to actuate said control. 5.The actuator of claim 4 wherein said valve is held in a first positionby pressure returning said impact member, the valve effective to admitreturn fluid to said impact member and to vent said follower drive, andsaid vent effective, through relief of return pressure, to operate saidvalve to a second position to apply hydraulic pressure to and controlsaid follower drive.
 6. The actuator of claim 5 wherein said vent isalso effective to open a further vent of said impact member return. 7.The actuator of claim 5 wherein said valve is operative in said secondposition to apply the pressure of said follower drive to said valve uponbuild-up of pressure in said follower drive associated with saidfollower reaching said impact member, said pressure operative to reversesaid valve to said first position.
 8. The actuator of claim 4 whereinsaid trigger is formed by mating portions of said follower and impactmember providing a seal when seated and creating pressure conditionswhereby said impact member and follower are returned as a unit, and inthe return position said vent operative to break said seal and expose asurface of said impact member to forces concelling return forces.
 9. Theactuator of claim 8 wherein said follower and said impact member whenseated together define a fluid volume, said fluid volume vented througha passage in said follower to a low pressure region, said followerhaving a surface portion exposed to hydraulic pressure effective topress said follower and impact member together and prevent reliefthrough said passage of pressure acting upon said impact member, andsaid vent operative to introduce hydraulic pressure to said fluid volumetending to break said seal.
 10. The actuator of claim 1 wherein saidfollower telescopes upon said impact member in response to said followerdrive.
 11. The actuator of claim 10 in the form of a linear actuatorwherein said impact member comprises a reciprocable piston having anenlarged middle portion, said follower compRises a sleeve linearlytelescopically fitted about the exterior of said piston, having a flaredend engageable upon said enlarged middle portion of said piston andhaving an oppositely directed surface exposed to said hydraulic followerdrive.
 12. The actuator of claim 11 including a housing forming a firstchamber into which said piston and flared end of said sleeve extend, afirst hydraulic line connected thereto for said hydraulic return, asecond chamber to which said oppositely directed surface of saidfollower is exposed, a second hydraulic line connected thereto for saidfollower drive, a third chamber intermediate said first and secondchambers and normally sealed therefrom, a third hydraulic line connectedthereto for venting, a passage extending from said third chamber to thespace between said flared end of said sleeve and said enlarged portionof said piston, and a vent passage in the exterior of said sleevepositioned so that when the sleeve is in its return position, said firstchamber is vented to said third chamber.
 13. The actuator of claim 12including a positive displacement, constant flow hydraulic pump and avalve effective alternately to direct flow to said second line andexhaust from said first line and vice versa.
 14. The actuator of claim13 wherein a branch from the outlet of said pump is connected so thatpressure therefrom urges said valve toward a first position directingsaid flow to said first line and exhaust from said second line, saidvalve constructed so that relief of said pressure on said branch linecauses said flows to be reversed.
 15. The actuator of claim 1 in theform of a linear actuator wherein said impact member comprises a linearpiston, a stationary tube in which the inner end of said piston isfitted, the inner end of said piston and the interior of said tubecooperating to form a volume for an expansible gas drive and saidfollower comprising a sleeve slidably engaged upon the exterior of saidtube and forming therewith a hydraulic chamber for said follower drive.16. The actuator of claim 1 in the form of a linear actuator whereinsaid impact member comprises a linear piston, said follower comprising asleeve slidably engaged upon the exterior of said piston, said housingand a portion of the side of said piston forming a hydraulic chamber forsaid follower drive, said piston extending beyond said chamber in sealedrelation into a further chamber, the end of said piston disposed in saidfurther chamber and exposed therein to an expansible fluid drive. 17.The actuator of claim 1 in the form of a linear actuator wherein saidimpact member comprises a linear piston including a housing defining abottoming position for said impact member in the absence of a workpieceat the impactreceiving position, said follower and housing cooperativelyconstructed to vent said follower drive prior to said follower reachingsaid impact member in said bottoming position, thereby preventing saidfollower from reaching the position for enabling application of returnforces to said piston.
 18. The actuator of claim 16 wherein said furtherchamber is connected to a supply of makeup fluid through a check valvethat permits inflow but not outflow to said further chamber, and a ventline from said further chamber that is exposed when said impact membertravels beyond a position predetermined to limit the maximum fluidtrapped in said further chamber.
 19. An actuator capable of deliveringrepeated impacts to a variable position comprising the combination of animpact member movable back and forth relative to said position, anexpansible fluid drive for driving said member toward said positionwhile expanding, and a return for returning said impact member andrecompressing said drive, said drive consisting of a volume of fluidconnected through a check valve to a supply of makeup fluid to allowinflow to said volume of fluid but not outflow from said volume offluid, and a vent from said volume of fluid opened by Said impact memberupon overtravel beyond a predetermined position and closed by saidimpact member upon return travel back to said predetermined positionsaid vent thereby being adapted to vent excess fluid at low pressurebefore it is recompressed by return motion of said impact member wherebysaid vent is effective to limit the maximum quantity of fluid trapped assaid volume of fluid.